Cement is the most vital building material that binds almost every structure that humans build today from the simplest of a concrete slab to engineering marvels.
It is an ever-present constituent in concrete which is made by mixing it with aggregate comprising sand, gravel, and water. This paste hardens over time and gives the concrete structure its required shape and strength and durability.
Cement use finds mentioned in the thousands of years-long history from the ancient Vedic age to Roman civilization. But unlike the binding material used then, the cement we use now is completely different. In today’s time, Portland cement is the most basic type of cement in general use around the world.
There are several raw materials used in the production process of this material. Minerals of natural origin as well as industrial products are being used for cement production. There are some main constituents as well as additives that help give Cement its usual properties.
Limestone is the most common ore containing calcium carbonate as raw material for cement manufacturing. Similarly, chalk which is a soft sedimentary rock as well as marl, which is a mixture of silica, clay substance, and iron oxide, is also used in cement. Deciding on the right proportion of lime is important because its excess causes expansion and disintegration of cement while deficiency decreases strength forcing cement to set undesirably fast.
The argillaceous constituent of raw mix, clay is formed by hydrous aluminum silicates. They play a major role in imparting strength to concrete. Excess silica prolongs the setting time of cement.
In a case where primary components needed in cement raw mix are absent in desirable proportions, corrective materials are used as additives. For example, Iron Oxide imparts color to cement. It helps in the fusion of raw materials as well. Magnesium Oxide imparts strength to the cement when mixed in a small quantity. Sulfates and Chlorides are also used in cement production with alkalies as important additives. Cement is also blended with some other materials like fly ash, ground blast furnace slag, and silica fume.
Cement manufacturing is a complex process and involves a series of steps. Each one plays an important role in determining the overall quality of the finished product.
There are six main stages of the cement manufacturing process.
- Raw material extraction: Raw ingredients needed for cement production are limestone, sand and clay, fly ash, shale, and of course bauxite. Ore rocks are quarried and crushed into bits of smaller pieces. Thereafter, the ingredients are prepared for heat-processing.
- Grinding and Blending: Crushed raw ingredients mixed with additives inside a kiln and powdered to a fine homogenous mixture. The composition of the raw mix is proportioned depending on desired properties. Generally, 80% of it is limestone components and 20% clay. First, the mixture is dried to reduce moisture content and then blended in rotating tables before the roller crushes it to a fine powder.
- Pre-Heating: The raw cement mix is turned into oxides inside a pre-heating chamber. Here, a series of cyclones utilize the hot gases produced from the kiln to heat the mixture before it is put into the kiln. The process reduces energy consumption and makes cement production more environment-friendly.
- Kiln Process: This is the principal stage of the cement production process. Here, the raw mix transforms into clinker after a series of chemical reactions between calcium and silicon dioxide compounds.
- Cooling and grinding: Once it exits the kiln, the clinker is rapidly cooled down with airflow. During this stage, several additives are combined with the clinker before it is ground to produce the final product, cement. In the final last stage of cement production, the cooled clinker product is transferred to rotating drums and ground into fine powder.
- Packing: Cement now finally produced is first conveyed from grinding mills to large storage tanks called silos where it is packed in certain size bags.